This invention relates to constant-force springs and especially to constant-force springs of the liquid-gas type.
Constant force springs may be used in gun recoil systems. The basic functions of any gun recoil system is to provide a means of:
A. Absorbing or converting the energy imparted to the recoiling mass by the fired ammunition, which is necessary in order to minimize loads within the supporting structure.
B. Restoring the recoiling mass to a condition similar to the condition that existed before firing, in order that a next shot may be fired.
An optimal recoil system would best satisfy the above objectives without compromising the constraints of low total weight, short cycle time, and reproducibility.
First of all, an optimal recoil system would store recoil energy rather than absorb (or dissipate) it. This would allow all of the stored recoil energy to act in rapidly restoring the gun to the state which existed before firing; and, also eliminate design and maintenance problems associated with special energy dissipators.
However, this proposed reduction of dissipative elements within a dynamical recoil system requires a greater reproducibility of the force-displacement characteristics of the energy storage device (spring). Therefore, an optimal recoil spring would have both low dissipation and high reproducibility.
Secondly, since mechanical energy is stored by the action of a force over a distance, the most efficient manner in which to store mechanical recoil energy is to apply a constant force to the recoiling mass. A constant force allows for the fastest absorbtion and retrieval of recoil energy for both a minimum peak braking force and a minimum stroke length. Furthermore, this approach reduces the strength requirement (and weight) of the supporting structure.
In summary then, an optimal general recoil system provides for three basic qualities:
A. Energy storage by the application of a constant force.
B. Reproducible constant force from one firing cycle to the next.
C. Low energy dissipation.
One particular application of a spring to a fire-out-of-battery gun requires a constant force spring whose force can be adjusted as a function of elevation angle.
This sensitivity of the applied force to an adjustment would also provide a means of compensating for changes in a recoil spring's operating characteristics. Therefore, a fourth requirement of an optimal recoil spring becomes:
D. Rapid adjustment in the applied force with changes in elevation angle and operating characteristics.
The fire-out-of-battery approach is used to greatly reduce the required recoil brake loads. The recoiling mass is caused to move forward before a round is fired. The round is then fired during this forward motion, and the firing energy acts to cancel and reverse the momentum of the recoiling mass. In essence, recoil brake loads are applied before firing and act to store kinetic energy within the recoiling mass. Acquired momentum then acts to cancel part of the firing impulse so peak/brake loads are decreased. Proper phasing and timing require a great reproducibility throughout the firing cycle. Gravity forces on the recoiling mass would vary with elevation angle and would add bias forces that the recoil spring must counter. In order that the reproducibility requirement of the overall system be met, the recoil system must rapidly adjust to these new conditions.
The above requirements of reproducibility and constant force may be met by a fluid/piston type of spring. Constant force can be achieved by a constant pressure; reproducibility would be as precise as the precision at which the constant pressure can be maintained.
A fluid/piston spring built as an all-liquid spring could be used in a closed volume by exploiting bulk modulus (compressibility) characteristics of the liquid. Readily available servo-controlled, positive displacement pumps would provide force adjustability. The principle drawback to an all-liquid spring is that it would require a fairly large volume of fluid (and weight). This large volume of liquid would be necessary to keep the spring rate low in comparison to the bias force (e.g., an almost constant-force spring).
A gas spring, however, would provide a highly reproducible constant force in a low volume. The several special problems associated with an all-gas spring are as follows:
1. Requires more elaborate seals than liquid springs.
2. Requires a more elaborate pumping system for pressure maintenance and force adjustment.
3. Presents a greater safety hazard if the pressure container should fracture. Gaseous expansion would tend to spew metal fragments. In a liquid spring, the liquid would lose pressure rapidly without causing fragments to gain high velocity. In the liquid-gas spring the liquid volume would act to dampen gaseous expansion.
The solution to the problem of obtaining all desired characteristics at a low total weight is found in a trade-off of characteristics as found in a liquid-gas spring. This approach involves a sealed bladder of gas within a liquid medium. The liquid is pumped for adjustability within the total liquid-gas volume, while the gas provides for most of the compressibility.
The liquid-gas recoil spring satisfies the description of an optimal recoil spring device. The proposed spring combines the advantages of both the all-liquid and all-gas springs with a minimum of the disadvantages.